package com.example.test.a20240530;

import java.util.Scanner;

public class HJ100等差数列 {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        final int i = in.nextInt();


        int sum = (int) (2 * i + 1.5 * i * (i - 1));
        System.out.println(sum);
    }
}
